The invention relates to a device including a circuit arrangement for supplying electric power to a load connected to the device in a power-on mode, comprising a driver stage, and a control unit adapted to supply turn-on pulses to the driver stage, and an output stage including a switching transistor, which switching transistor can be turned on by the driver stage in the power-on mode when turn-on pulses occur, to assume a conductive switching state for a time interval, and can be turned off after expiry of the time interval, to assume a non-conductive switching state, the driver stage being adapted, in addition, to monitor the operating temperature of the switching transistor and to activate a power-off mode, which mode is activated when the operating temperature of the switching transistor exceeds a first limit temperature, in which power-off mode the switching transistor is permanently driven into its non-conductive switching state by the driver stage in spite of the occurrence of turn-on pulses.
Such a device of the type defined in the opening paragraph is known from, for example, the document DE 38 43 277 A1. In this known device the circuit arrangement includes a control unit comprising a current sensing resistor and a clock generator, which unit supplies turn-on pulses to the driver stage and when the turn-on pulses appear the switching transistor can be driven into a conductive switching state by the driver stage for the time in which the turn-on pulses are high in the power-on mode.
During operation of the known device the switching transistor of the output stage assumes an operating temperature. In a fault condition, in which for example the switching transistor conducts permanently or in which the load connected to the device has been short-circuited, the operating temperature of the switching transistor rises strongly, so that there is a risk that the switching transistor reaches or exceeds a maximum permissible temperature, as a result of which the switching transistor is damaged or even destroyed. In order to preclude this, the driver stage has been provided, which stage serves for monitoring the operating temperature of the output stage as well as for activating the power-off mode when the operating temperature of the switching transistor exceeds a first limit temperature which is lower than the maximum permissible temperature. To activate the power-off mode the driver stage includes a temperature-dependent resistor, i.e. an NTC resistor having a negative temperature coefficient. The NTC resistor is connected to the switching transistor via a thermally conductive connection, as a result of which the NTC resistor assumes the instantaneous operating temperature of the switching transistor. The NTC resistor has one end connected to an output of the control unit and has its other end connected to a second resistor of the driver stage, with which second resistor it forms a temperature dependent voltage divider, to which the turn-on pulses of the control unit can be applied and which has its tapping connected a control input of a switching stage, which is likewise connected to the output of the control unit and which comprises two transistors in a thyristor-like arrangement. Depending on the instantaneous temperature-dependent resistance ratio of the voltage-divider resistors, the switching stage can be switched either to a non-conductive switching state corresponding to the power-on mode, or to a conductive switching state corresponding to the power-off mode.
When the operating temperature of the switching transistor rises in the case of a fault condition occurring in the power-on mode, the NTC resistor is strongly heated via the thermally conductive connection, as a result of which the resistance value of the NTC resistor decreases. When the operating temperature of the switching transistor exceeds a first limit temperature the resistance of the NTC resistor decreases, thereby causing the switching stage to be driven into its conductive switching state owing to the resistance ratio between the resistors of the voltage divider and the turn-on pulses supplied to the driver stage by the control unit being drained to a reference potential via the switching stage, so that the power-off mode is then activated in the known device, in which mode, in spite of the occurrence of switching pulses from the driver stage, the switching transistor is not turned on but is permanently driven into its non-conductive switching state, as a result of which only a low or no power at all is supplied to the load.
In the power-off mode, in which the switching transistor is in its non-conductive switching state and, consequently, no switching loss is produced in the switching transistor by a change of in the switching state, the operating temperature of the switching transistor and hence the temperature of the NTC resistor decreases as a result of cooling. When the temperature of the switching transistor falls below a second limit temperature, the resistance of the NTC resistor increases, causing the switching stage to be turned off owing to the resistance ratio between the resistors of the voltage divider, so that the power-on mode is restored.
It has been found, however, that in the known device the operating temperature of the switching transistor in the event of a fault can rise rapidly to such an extent that the activation of the power-off mode by the temperature-dependent resistor, i.e. the NTC resistor, connected to the switching transistor by the thermally conductive connection, is effected too late and the switching transistor is consequently destroyed by overheating before the temperature-dependent resistor has reached the first limit temperature. Moreover, it has been found that in the known device at the end of time interval in which it is in its conductive switching state the switching transistor is switched only comparatively slowly to its non-conductive switching state by the driver stage, as a result of which the power dissipated in the switching transistor owing to the change in switching state is comparatively high and the operating temperature of the switching transistor is permanently raised in operation. Such as permanently raised operating temperature of the switching transistor, however, reduces the life of the switching transistor, which is undesirable.